How to Measure Power Supply Output Ripple

If you don't measure ripple correctly, you might be fool by the instrument.

Testing for output ripple voltage and compliance of a Power Supply Unit (PSU).

Units of Measurement

Ripple is commonly measured in millivolts peak to peak (mVp-p) for most ATX computer applications. This is the measurement of the amplitude of the waveform from its highest voltage to its lowest voltage in a given frequency or time period.

Tools and test instruments required

An oscilloscope that has been properly calibrated is going to be the most powerful tool used for these test. Make sure that it has the ability to limit the bandwidth of the test channel to 20MHz. By doing this, you will eliminate a lot of excess noise that would otherwise be picked up on the scope but would not affect the actual performance of the PSU output in reality. The scope probes should be of high quality and have the ability to be outfitted with a tip (+) ring (-) lead (see figure below).

Using a standard ground lead is unacceptable due to the added noise introduced by the ground clip and wire.

The Load Board (LB) is where everything comes together (test equipment, PSU and the load) which makes it imperative that all connections are solid, all capacitors and tip and ring test points (TP) are soldered properly and all wiring going from the PSU to the LB and the LB to the loads are neat and orderly. We will discuss proper LB configuration later.

The actual load you are using to test the PSU will affect the outcome of your test as well. You will notice that some electronic loads will have such configurations as constant current, constant resistance and constant voltage. Since most voltage outputs sag or rise during different load configurations, the most ideal set-up is an electronic load in constant current mode. If you do not have access to a constant current electronic load, a purely resistive load made up of a network of high wattage resistors will do.

Finally, the source of AC voltage for your PSU will come into play. The best configuration is an isolated transformer (1:1) in order to decouple your test instruments from the PSU device under test (DUT). The ability to increase or decrease the input AC voltage with a variac will increase the scope of your testing ability by stressing the primary components of your DUT outside of its normal operating range of 110VAC (i.e. 90~132 VAC). Larger test facilities will use an electronic power source in which voltage, period and AC waveform shape can be manipulated through test programs or in real time.

Test Set-Up

The PSU will be powered by the variac and isolation transformer.

PSU output is plugged into the LB.

The LB is hard wired to loads.

In order to find the worst case scenario, PSU input power and output power should be varied through its rated range for all outputs (see fig below).

ATX PSU 12VCPU TEST CONDITIONS

Channel Under Test

VAC input

12V_IO

3.3V

5.08V

5.08V_STBY

-12V

12VCPU minimum load

90 VAC

min load

min load

min load

min load

min load

115 VAC

nom load

nom load

nom load

nom load

nom load

132 VAC

max load

max load

max load

max load

max load

12VCPU nominal load

90 VAC

min load

min load

min load

min load

min load

115 VAC

nom load

nom load

nom load

nom load

nom load

132 VAC

max load

max load

max load

max load

max load

12VCPU maximum load

90 VAC

min load

min load

min load

min load

min load

115 VAC

nom load

nom load

nom load

nom load

nom load

132 VAC

max load

max load

max load

max load

max load

It should be noted that most switching power supplies will exhibit higher ripple when sampling from either low frequency (120Hz) and/ or switching frequency of the secondary FETs (20 to 300kHz), so a sweep of frequency is required to expose the PSU worst case scenario.

You will record all ripple conditions in the red boxes in high and low frequency ranges to determine the worst case scenario for the particular output of the DUT.

The LB should have added capacitance to create a filter to cancel out any noise that will be introduced by the length of wire from the loads to the LB, noise generated by the loads themselves and the length of wire used on the test leads. On most ATX set-ups, a low ESR 10uF cap (electrolytic or tantalum) should be in parallel with a .47uF ceramic. These capacitors should be hard wired as close to the TP as possible.

These tests will be repeated for each output in order to assure compliance with Intel ATX specifications.

While testing, be sure that you do not exceed total rated output of the PSU. Some ratings in PSU specs are peak surge output capacity and if all outputs are brought to this level at the same time damage may occur to the power supply, test equipment and yourself.